There are bearings used for automotive wheels, i.e., for driving wheels and follower wheels, which will be hereinafter referred to as automotive hub unit bearings, where a hub ring for fixing an automotive wheel is rotatably supported via a double-row rolling bearing. For the structural reason, the inner ring is generally rotated in the automotive hub unit bearing for driving wheel; and either the inner ring or the outer ring is rotated in that for follower wheel. The automotive hub unit bearings are roughly divided into four groups: the first generation structure where a bearing for automotive wheel such as a double row angular rolling bearing or the like is fitted into a space between a knuckle for constituting a suspension and a hub ring; the second generation structure where a flange for fixing a car body or a flange for attaching a wheel is directly formed on the periphery of an external member; the third generation structure where one of the internal raceways is directly formed on the circumference of the hub ring; and the fourth generation structure where the respective internal raceways are directly formed on the circumference of the hub ring and the circumference of the external joint member of a constant velocity universal joint coupling.
Motorcars are often transported by train or truck. During the transportation, fretting wear may occur on the rolling raceways of bearing due to vibration induced by rail joints and rough roads. The fretting is a surface damage induced under a minute amplitude of vibration, and generates oxidized debris in the air, which is said to often cause abrasive action to lead to considerable corrosion (Yuji Yamamoto et al., “Tribology” Rikogakusha Publishing Co., Ltd. (25 Nov. 1999): 201-203).
Some measures are proposed to prevent the fretting wear, for example, (1) decreasing the relative amplitude of sliding motion, (2) separating the contact surfaces not to come in a direct contact therewith, and (3) coating the contact surfaces with a phosphate coating or the like or supplying a lubricating oil or grease to prevent the metal surfaces from adhering to each other by (Yuji Yamamoto et al., “Tribology” Rikogakusha Publishing Co., Ltd. (25 Nov. 1999): 201-203).
Many greases with excellent anti-fretting properties have been developed and examined especially in the field of grease for automotive hub unit bearings.
For example, it is reported that the grease employing an aromatic diurea as a thickener to increase the grease consistency exhibits significantly improved anti-fretting properties when compared with the grease containing a conventional lithium soap as the thickener (Yuji Yamamoto et al., “Tribology” Rikogakusha Publishing Co., Ltd. (25 Nov. 1999): 201-203).
JP 2006-77056 A discloses that a grease comprising a particular urea compound and organic molybdenum compound can significantly extend the flaking life and the lubricating life and reduce the fretting.
JP 2003-321693 A reports that improved anti-fretting properties can be obtained by adding to a lubricating base oil a particular diurea compound and at least one selected from the group consisting of oxidized paraffins and phosphorus-containing compounds at particular concentrations.
As mentioned above, methods for improving the grease flowability by choosing the kind of thickener or increasing the consistency, and methods of adding the particular additives have been practically employed to prevent the problem of fretting.
However, any of the above-mentioned methods are measures taken against the fretting which may occur at the normal temperature, and the effects of those methods are still insufficient to prevent the fretting from occurring at low temperatures.
Under the conditions of low temperatures, the grease flowability decreases as the viscosity of the base oil contained in the grease increases, so that the possibility of preventing the metal surfaces from adhering to each other by the supply of grease thereto as mentioned in the above method (3) will be considerably lowered. Therefore, the effects of the additives can hardly be expected. In light of the above, it is proposed to use a synthetic oil characterized by a relatively moderate increase in viscosity even at low temperatures. However, such a synthetic oil has not yet been widely used from the economical viewpoint.
The automotive hub unit bearing is disposed at a portion readily exposed to muddy water and the like. Therefore, a sealing apparatus such as a seal is used to tightly seal the space between the external member and the internal member.
However, it is difficult to keep a perfect sealing condition over an extended period of time. Deterioration of the sealing apparatus may let a slight amount of muddy water permeate into the inside. The muddy water thus induces rust on the bearing, which will contribute to a decrease of the bearing life.
Accordingly, the grease for hub unit bearing is required to have both anti-fretting properties and water resistance, in particular, excellent rust preventing properties.